Fred (60 kg) is rollerblading at a velocity of 25 km/h [E] when he sees a broken glass bottle on the path ahead. He brakes and is slowed to a velocity of 8 km/h [E] in 4.2 s. What is the coefficient of friction between Fred's rollerblades and the ground? (Hint: Find FN and Ff first.)

Question

Fred (60 kg) is rollerblading at a velocity of 25 km/h [E] when he sees a broken glass bottle on the path ahead. He brakes and is slowed to a velocity of 8 km/h [E] in 4.2 s. What is the coefficient of friction between Fred's rollerblades and the ground? (Hint: Find FN and Ff first.)

Answer 1

First find Fred's acceleration:

a = (2.2-6.9 / 4.2)
a = -1.12 m/s^2 [W]

Find the coefficient of friction (μk):
μk = Ff/Fn

We have to find Ff and Fn first:
Ff = ma
Fn = mg

Shortcut:
Therefore we can say μk = a/g
μk=-1.12/9.8
Therefore, μk=0.11.